I'm trying to install ubuntu server 12.04 on an older desktop for a friend. The tower doesnt have an onboard graphics card, so we had to install an AGP graphics card to plug in a monitor. The install all displayed correctly, but after completing, the monitor switched to "No Signal". I was able to boot into Ubuntu initially by pressing enter on this invisible screen, and the OS seemed to display correctly.

I tried holding down SHIFT, after modifying /etc/default/grub to have a timeout, to no avail.